Since we're downloading arbitrary executable code from websites and
running it, we should at least have some confidence it's the right
thing we're running.
It's nice to be able to build from source control HEAD, and
especially for packages under than Chocolate Doom itself. As a start,
define "latest" variants for all the SDL2 packages.
The only place this is needed is when determining the directory name
that the tar file will be extracted to. This can be done by examining
the .tar file name without needing to know the version itself.
These modules encapsulate individual pieces of functionality and make
the code far more maintainable. This is a pretty big refactoring but
overall a big win.
